The present invention relates to a drive force distribution device and a method for distributing drive force.
A typical drive force distribution device is capable of changing a drive force distribution ratio of a main drive wheel to a sub drive wheel. Typically, a torque coupling is provided in a drive force transmission system. Operation of the torque coupling is controlled in such a manner as to change the torque transmission rate (the transmitted torque) from an input to an outlet of the torque coupling. This regulates the drive force distribution ratio of the main drive wheel to the sub drive wheel.
The torque coupling employs a friction clutch, which generates heat through frictional engagement of clutch plates. Also, a transfer case or a differential, which are arranged in the drive force transmission system of a vehicle, generate heat through friction caused by engagement of gears. Overheating of these heat generating portions thus must be suppressed.
For example, Japanese Laid-Open Patent Publication No. 2003-136990 describes a drive force distribution device that detects the temperatures of a differential or a transfer case provided in a drive force transmission system. If the temperature of the differential or the transfer case exceeds a corresponding predetermined level, the drive force distribution device controls operation of a torque coupling in such a manner as to suppress overheating of the differential or the transfer case.
However, to deploy a temperature sensor in each of the heat generating portions, an increased cost is needed to prepare, assemble, and wire the necessary parts. Thus, to avoid this problem, Japanese Laid-Open Patent Publication No. 7-12155, for example, describes a method for estimating the temperature of a torque coupling in correspondence with the rotational speeds of an input shaft and an output shaft provided in the torque coupling, and the torque transmission rate. By employing the method, overheating of the heat generating portions is effectively suppressed through a simplified structure.
Specifically, to estimate the temperature of each heat generating portion in correspondence with the rotational speed of the torque coupling and the torque transmission rate, heat generating energy accumulated in the torque coupling is determined basically in correspondence with the load acting on the torque coupling. The heat generating energy is then accumulated. However, in this case, the accumulated heat generating energy, or the estimated temperature of the torque coupling, is cleared (deleted) once the engine, or the drive source, is stopped, or the ignition is turned off.
The temperature of the torque coupling, which rises when the engine operates, drops when the engine is held in a deactivated state. When the engine is re-started, or the ignition is turned on, estimation of the temperature of the torque coupling is resumed. If the temperature of the torque coupling has not decreased sufficiently in the engine deactivation period, there may be a difference between the actual temperature of the torque coupling and an initial value set for the temperature estimation.
Specifically, for example, if the time from deactivation of the engine to restarting of the engine is relatively short, the actual temperature of the torque coupling may remain relatively high when the engine is re-started, indicating necessity of suppression of overheating. However, in this case, the temperature of the torque coupling may be determined to be lower than the actual level. This may cause increased load to act on the torque coupling continuously, making it impossible to effectively protect the torque coupling.